1. Field of the Invention
This invention pertains to a process for producing transuranic isotopes and more particularly to a process for producing ultra-pure Plutonium-238 (Pu-238). The ultra-pure Pu-238 is especially useful as a heat source in Radioisotope Thermal Generators (RTGs) in the National Aeronautics and Space Administration (NASA) space program and may also be used in Department of Defense (DOD) space applications.
2. Description of the Related Art
Radioactive isotopes are well known in the art and various methods have heretofore been used to product isotopes of interest. One isotope of interest is Pu-238. It is well known to those skilled in the art that Pu-238 may be produced using either NP-237 or Am-241 targets.
In the former, irradiation of Np-237 targets in a high thermal neutron flux produces Np-238, which decays via β decay (2.12 days) into Pu-238. This is shown as: This has been the method of choice at the Department of Energy's Savannah River Site (SRS) production reactors. However, there are several shortcomings with this method of production. First, the production efficiency is quite limited, i.e., to approximately 13% efficiency. This is seen from the fact that the Pu-238 produced in the target after only 2.12 day's half-life decay, itself becomes a target for production of higher isotopes of plutonium, thus reducing the Pu-238 purity by producing Pu-239 and Pu-240. Second, this process produce a hazardous Pu-236 by-product. As noted on the decay chain, above, there is a η→2η or γ→η reaction that results in the production of Uranium-236 (U-236) and Pu-236. These reactions increase with exposure to fast neutron flux. Pu-236 decays to U-232, which has a hazardous gamma-ray energy emitting daughter product. Even a few parts per million U-232 increase the radiation exposure hazard to personnel dramatically. Lastly, Np-237 must be chemically purified before target fabrication. This is seen from the fact that Np-237 decays to Protactinium-233 (Pa-233), which in turn has a strong gamma-ray emission with its beta decay to U-233 (half-life 27 days). Therefore, the Np-237 was stored in solution at SRS and chemically processed immediately before fabricating targets. Solution storage of Np-237 may not be practical at an alternate production site.
In the alternate production method, Am-241 targets are irradiated with high-energy neutrons to produce first Am-242 which decays via β decay (16 hours) to Cm-242, which in turn decays via α decay (163 days) to Pu-238. This is shown as: Production of Pu-238 via this process resulted in extremely low production efficiency, and more importantly, low Pu-238 purity, i.e., only about 72% pure Pu-238, having a mixture of radioisotopes of Pu-242 and other Pu isotopes. This extremely low yield of Pu-238, as well as the mixture of unwanted radioisotopes makes this production route of limited use in the production of Pu-238 for applications such as a heat source in Radioisotope Thermal Generators (RTGs) in the NASA's space program. There is a need to find an efficient process for producing Pu-238 wherein the produced Pu-238 is in ultra-pure form, i.e., 95% or higher.
It is an object of this invention to provide a highly efficient and low-cost process for producing Pu-238. Another object is to provide a method for producing ultra-pure Pu-238.